The discovery of dark matter is of fundamental importance to cosmology, astrophysics and elementary particle physics. A broad range of observations from galactic to cosmological scales indicate that nearly 90% of the matter in the universe is made up of some new type of particle, different from ordinary matter. This type of matter has revealed itself only through gravity and is referred to as "dark matter" because it neither emits nor absorbs light. A leading hypothesis is that the dark matter is comprised of Weakly Interacting Massive Particles, or WIMPs, that were produced shortly after the Big Bang from annihilation of ordinary matter. If WIMPs are the dark matter then they will make up a spherical halo around the Milky Way and be detectable through interactions with atomic nuclei in a suitably massive and sensitive detector. This award will further the search for WIMPs by supporting work on the LUX (Large Underground Xenon) dark matter experiment that is currently operating in the Homestake Mine in the Sanford Underground Research Facility in South Dakota. This award will also support development of the followup LZ (LUX-Zeplin) experiment, with a proposed mass of seven tons. The search for dark matter has been driven by the development of radiation detectors with ultra-low radioactive backgrounds. These detectors are also playing a role in neutrino physics, and have the potential for security, medical and industrial applications. Work on LUX and LZ provides excellent and unique opportunities for student training with technical skills that can be used in careers in basic research or in the private sector.
Using 250 kg of actively monitored liquid xenon, the LUX experiment is now at the vanguard in the search for dark matter. The upper limit on the interaction rate between WIMPs and ordinary matter that is now established by LUX rules out new regions of parameter space, and provides solid evidence that claims of WIMPs in the low mass region around 10 GeV are likely due to instrumental effects or unidentified backgrounds. With the next few years of operations and analysis it will become clear whether WIMPs will be detected in LUX or new upper limits set. In either case, LZ will be well positioned to follow up and continue the search with sensitivities 1000 times better than LUX's current limit.
